During the treatment of annealing of the glass, following casting of the glass ribbon an effort is made to reduce the stresses as may be present in the glass ribbon to acceptable values. The annealing treatment should also make it possible to eliminate the risks of fracture of the glass ribbon, and particularly the "snaking" phenomenon at the moment of cutting. The "snaking" phenomenon is particularly difficult to control when it occurs.
It is well known that the temperature of a glass ribbon may be rebalanced by localized heat treatments to reduce stresses. However, to accomplish this end it is necessary to take a measurement of stresses as quickly as possible so that corrective treatment may be made continuously and as soon as possible along the path of movement the glass ribbon.
Knowledge of any permanent stresses as may exist in a float glass ribbon is important to permit the optimizing of the cut. Thus, knowledge of the permanent stresses in the float glass ribbon makes it possible to select a cut which will achieve a maximum number of pieces of glass in which the stresses are acceptable. In other words, knowledge of permanent stresses makes it possible to reduce the part of the glass produced that does not meet a set standard.
Traditionally, optical means have been employed to measure stresses on the float glass ribbon. The presence of stresses on the float glass ribbon is revealed by the appearance of a double refraction of the glass. Detection and measurement of this double refraction is performed by a method of using polarized light and serves to determine the stresses in the glass.
According to one known method, a polarized light beam is passed through both the glass ribbon and a quarter-wave plate. Any stresses in the glass ribbon will result in a transformation of the polarized beam. The polarized beam, then, is passed through either a second polarizer or analyzer to a light responsive cell which transduces the light signal to an electric signal.
One polarizer is driven in rotation relative to the other causing the light signal to periodically vary in intensity. The quarter-wave plate is located between the polarizer that is driven and the glass ribbon under test. The presence of the birefringent glass is shown by a phase difference of intensity variations in relation to those corresponding to an isotropic glass (or the absence of glass). It is shown that the phase difference observed depends directly on the stresses present in the ribbon analyzed according to a law whose mathematical expression is: EQU .alpha.=P.times.C.times.d.times. /.gamma. (1)
wherein:
.alpha. is the angle of measurement of the phase difference expressed in radians;
P is a designation of stress;
C is a characteristic constant of the nature of the glass, called the photoelastic or Brewster constant;
d is the thickness of the glass through which the polarized light beam passes; and
.gamma. is the wavelength of the analysis beam.
The longitudinal stresses in float glass ribbons normally are very much greater than the crosswise stresses when the measurement is taken at a sufficient distance from the end of the glass ribbon. Consequently, the measurement is made only to determine longitudinal stresses. Experience has shown that the distribution of longitudinal stresses in a glass ribbon along a float glass production line remains approximately the same over relatively long periods, and measurements over the entire width of a glass ribbon whose duration is in minutes are well suited for the controls contemplated. For this reason, it is customary to use a mobile measuring unit driven in a back-and-forth crosswise movement. This unit and movement makes it possible to determine continuously the stresses over the entire width of the glass ribbon. A measuring unit of this type is described in U.S. Pat. No. 2,993,402 to Dunipace et al.
While it is possible to follow the described techniques to obtain a measurement of stresses, it has been found in practice that numerous imperfections can compromise the usefullness of the measurement. The imperfections may stem from the choice of the means used in obtaining the measurements. Thus, the means used may fail in precision or the means may not be sufficiently reliable. The imperfections may also stem from physical factors which have not been considered in the methods described and where variations influence the measurements.